checksrc: introduce 'banfunc' to ban specific functions

Use 'banfunc' and 'allowfunc' in .checksrc to specify which functions to
ban or allow to be used. This saves us from having to edi

checksrc: introduce 'banfunc' to ban specific functions

Use 'banfunc' and 'allowfunc' in .checksrc to specify which functions to
ban or allow to be used. This saves us from having to edit the script
going forward when we want to ban or allow specific functions.

This replaces a set of previous rules and all banned functions are now
checked with the BANNEDFUNC rule.

There is a set of default banned functions, shown by invoking
./checksrc.

Also, -a and -b options are added to specify allowed or banned functions
on the command line.

Closes #15835

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checksrc: add STRNCPY as an opt-in rule to detect and error on strncpy

make "lib/.checksrc" enable it

Closes #14830

lib: don't use strerror()

We have and provide Curl_strerror() internally for a reason: strerror()
is not necessarily thread-safe so we should always try to avoid it.

Extended ch

lib: don't use strerror()

We have and provide Curl_strerror() internally for a reason: strerror()
is not necessarily thread-safe so we should always try to avoid it.

Extended checksrc to warn for this, but feature the check disabled by
default and only enable it in lib/

Closes #7685

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